I believe that Luna has slightly more raw power than Celestia but Celestia knows how to control it as she spent so much time trainting with Starswirl. I think that when Luna is older (and no I'm not just saying that Celestia is stronger because she's older...) ...and she has MORE TIME TO PRACICE her magic *cough* *cough* this may be why she doesn't get enough screen time *cough* she may be an equal to Celestia if not, her better.
Yes,I do think that Luna has the potential to be more powerful than Celestia because she not only has this amazing raw ability to perfect but she has this darker side to her magic that I believe after "Do Princesses Dream of Magic Sheep?" she should now be able to bend to her will (she should be able to use NMM's magic as her own).
And also lots of you are saying that NMM is a separate entity to Luna but that is only in the comics (cannon B )n they called NMM part of the "Nightmare Forces" but in the show (cannon A) in "Do Princesses Dream of Magic Sheep?" Luna absorbed the TANTABUS THAT SHE CREATED!!!!!!! So basically, the show ITSELF is more of cannon than the comics aren't really a thing now, it's the TANTABUS and it's part of Princess Luna now.
THE LUNAR REPUBLIC IS AWESOME LUNA IS BEST PRINCESS!!!